So many of us head to those big-box discounters looking for a deal. But this one may be turning sour. It was last night here, we showed you those two rings side by side, one, a real a tiffany ring. The other one sold at costco with tiffany in the title. Tonight, tiffany is not happy, taking costco to court. Do they have a case? Here's abc's tanya rivero. Reporter: It's the little blue box versus the big-box retailer. Tiffany and company has slapped costco with a lawsuit. Showing these pictures show tiffany knockoffs inside a california costco. Tiffany sent someone in, bought one of the rings, and they weren't made by tiffany. They're not tiffany rings. They had nothing to do with tiffany. Reporter: Being sold at a fraction of what real tiffany rings cost. Everybody would love a deal on a tiffany ring. Unfortunately, it doesn't happen. Tiffany diamonds are never on sale. Reporter: The diamonds on the costco rings are real, even good quality, but tiffany said they're not tiffany diamonds and alleges that costco had been calling that for years. Thousands of engagement rings were sold using the tiffany trademark. Reporter: A rep for costco told abc news that we will be making no comment on this story. An unnamed costco story went to tiffany and blew the whistle. She was offended that tiffany would be selling their rings in costco. Reporter: Shoppers outside tiffany's flagship fifth avenue store sided with tiffany. No way around it, that little blue box will cost you and there's only one place to get it. Costco has since removed any mention of the name tiffany from its rings. But tiffany still wants costco to pay up three times what they made off those rings and tiffany wants costco to contact each person personally. When we come back here

This transcript has been automatically generated and may not be 100% accurate.